As crystalline thermoplastic resin films for electrical insulation, films made of polypropylene resins, polystyrene resins, polyester resins, and polyphenylene sulfide resins have been known, for example, and they have been used for film capacitors, flexible printed circuit boards, motor insulation, etc. The film capacitor is produced by combining such a crystalline thermoplastic resin film with an aluminum foil or like thin metal film, followed by winding or stacking. The flexible printed circuit board is produced by stacking the thin metal film on at least one side of the crystalline thermoplastic resin film to form a circuit, for example. A film for motor insulation is used, for example, as a wedge material or a slot material for insulation between a coil and a stator in a motor.
In recent years, electrically insulating materials for electric cars, hybrid cars, and the like are required to have higher withstand voltage characteristics.
As a polyester film for capacitors which has excellent heat resistance, moisture resistance, and electrical characteristics and is usable in a car engine compartment, for example, JP-A-2000-173855 (Patent Document 1) discloses the use of a polyethylene-2,6-naphthalate film having an intrinsic viscosity and a crystallinity within specific ranges.
Further, as a film for capacitors which has excellent dielectric characteristics and withstand voltage characteristics, JP-A-2005-289065 (Patent Document 2) proposes a thermoplastic resin film containing a large amount of a phenolic stabilizer. Specifically, it discloses the addition of a stabilizer during the production of a thermoplastic resin. Likewise, JP-A-2003-301039 (Patent Document 3) discloses the presence of at least one stabilizer capable of preventing oxidative decomposition in a concentration of 100 to 10000 ppm relative to a crystalline polyester. The Patent Document 3 is characterized in that the stabilizer chemically bonded to the crystalline polyester is present to reduce surface defects, whereby the contamination of a film production apparatus during production can be reduced. It is also disclosed that in order to form the chemical bond between the stabilizer to the polyester, a hindered phenol having a carboxyl group and/or an ester group is preferably mixed during a polycondensation reaction of the polyester and used.
As seen above, as a method for incorporating the large amount of a stabilizer into a film, it has been proposed to add the stabilizer in a stage of the resin polycondensation reaction. However, WO 2008/149869 (Patent Document 4) discloses a polyester film for electrical insulation which contains a titanium compound and a radical scavenging stabilizer in an amount of 1000 to 50000 ppm, and proposes that the proportion of the radical scavenging stabilizer chemically bonded to a polyester is not more than 200 ppm. The Patent Document 4 states as follows. When the radical scavenging stabilizer is added during film formation, the stabilizer does not react with the polyester resin in the resulting film. As a result, the radical scavenging ability of the stabilizer is enhanced, and also, in synergy with a titanium catalyst, withstand voltage characteristics higher than before can be obtained. Further, an initial withstand voltage is maintained even after a long-term heat treatment.
Patent Document 1: JP-A-2000-173855
Patent Document 2: JP-A-2005-289065
Patent Document 3: JP-A-2003-301039
Patent Document 4: WO 2008/149869